Multiple-antenna technology is widely used in the wireless communication systems. For Long Term Evolution (LTE), Downlink (DL) Multiple Input Multiple Output (MIMO) was standardized in Third Generation Partnership Project (3GPP) Release 8 (Rel-8). For Wideband Code Division Multiple Access High Speed Packet Access (WCDMA-HSPA) evolution, downlink 2 by 2 MIMO was specified in 3GPP Rel-7. The introduction of uplink multiple-antenna technology is being discussed in 3GPP, including Uplink Transmit Diversity (ULTD) in WCDMA and uplink (UL) MIMO in LTE. With ULTD or UL MIMO, there will be at least 2 transmit (TX) antennas in the UE.
Two well-known examples of ULTD that have been studied, e.g., within Rel-10 scope of 3GPP for WCDMA/HSPA, are Switch Antenna Transmit Diversity (SAID) and Beam Forming Transmit Diversity (BFTD). SAID and BFTD are here used as examples for illustration purpose only. For SATD, the UE transmits from one of its antennas, e.g., the TX antenna associated with the stronger channel (i.e. better estimated uplink quality). This can be estimated in a distributed fashion by the UE from the received UL TPC command sent by the base station. For BFTD, the UE transmits from both TX antennas with an estimated complex-valued weight factor (sometimes also referred to as pre-coding vector) to maximize received power or Signal to Interference plus Noise Ratio (SINR) at the radio base station/Node B, receiver, referred to as NodeB herein. The ULTD technology can improve the system capacity and coverage and save the UE battery consumption in most cases. There are two types of ULTD modes, Open Loop ULTD (OLTD) and Closed Loop ULTD (CLTD). In the latter case there is a specific downlink feedback channel from one or more Node Bs to the UE carrying the pre-coding vector or the information to assist the generation of pre-coding vectors. For OLTD, there is no specific downlink feedback channel from the Node B to the UE to signal the pre-coding information. For CLTD, in order for the Node B to generate the desired pre-coding information to the UE over the specific downlink feedback channel, the Node B should also monitor the uplink channels. In existing systems the pre-coding vector is typically generated in the Node B and sent to the UE. Alternatively the Node B can also send only channel state information to the UE, which in turn autonomously can decide the suitable pre-coding vector.
OLTD for WCDMA-HSPA
For WCDMA/HSPA, OLTD has been studied within the Rel-10 time frame in 3GPP. The UE performs transmit adaptation of the 2 transmit antennas based on the already existing information, e.g. the TPC commands. Examples of algorithms for open loop SATO and for open loop BF are described in 3GPP TR 25.863, “Uplink Tx Diversity for HSPA Study Item Technical Report”.
In case of SATO, there are typically two or more TX antennas and one single full-power power amplifier in the UE. With the scheme described in 3GPP TR 25.863, “Uplink Tx Diversity for HSPA Study Item Technical Report”, the UE selects the TX antenna according to the TPC statistic in accordance with:    1) Let TPC command DOWN be represented by −1 and TPC command UP by +1. Then let the UE accumulate all received TPC commands.    2) At each frame border the accumulated TPC sum is compared with 0. If the sum is larger than 0 the transmit antenna is switched.    3) If the same transmit antenna has been used for a number x consecutive frames the UE automatically switches antenna. The number x can be referred as the forced switch circle and determined according to the radio environments.    4) Every time an antenna switch occurs the accumulated TPC sum is reset to 0.
In case of BFTD, there are two power amplifiers in the UE. With the algorithm described in TR 25.863, “Uplink Tx Diversity for HSPA Study Item Technical Report” the UE adjusts the beam by adjusting the relative phase difference between the two antennas based on the received TPC commands:    A. The phase offset, δ, can be 48 degrees, ε can be 12 degrees.    B. Let TPC command DOWN be represented by −1 and TPC command UP by +1.            1. Initial relative phase between two transmitters Δφ=−δ/2 for the first slot (#1 slot). ε is kept zero until two TPC commands become available to the UE.        2. Apply relative phase for the next slot Δφ=Δφ+δ        3. Determine new relative phase:                    a. if TPC1>TPC2, Δφ=Δφ+ε            b. if TPC2>TPC1, Δφ=Δφ−ε            c. otherwise, no change            Note that TPC1 and TPC2 correspond to slot (1,2), (3,4), . . . (i*2−1, i*2), where i=1 to n.                        4. Apply relative phase for the next slot Δφ=Δφ−δ        5. Go to step 2        
A UL OLTD capable UE can in principle be configured in default mode (fixed single TX antenna transmission), OLTD mode (open loop SAID and/or BFTD). The OLTD mode can be supported by a cell belonging to a legacy, Node B.
CLTD in HSPA
CLTD was proposed in 3GPP RAN1-61 meeting, see 3GPP R1-102931, “Concept of UL Closed Loop Transmit Diversity”, Huawei. In this paper, the uplink closed loop transmit diversity scheme is based on explicit uplink channel estimation and Channel Status Information (CSI) feedback. Thus, the network controls the UE behavior and decides the weights (pre-coding vector) applied by the UE at a given time, Simulation results show that the average throughput gain reaches 14% in Pedestrian A channel (3 km/h) and up to 10% in Vehicular A (30 km/h) channel. Overall CLTD can thus be a feature that can be further considered for improving the HSPA uplink. As a matter of fact CLTD is being standardized by 3GPP in Rel-11 of WCDMA/HSPA.
A UL CLTD UE can be configured in default TX antenna mode, open loop BFTD Mode and closed loop BFTD mode. The first two TX antenna modes can be supported by legacy Node Bs. The last TX antenna mode (closed loop BFTD) may however not be supported by a cell belonging to a legacy Node B since, e.g., a new feedback channel needs to be implemented at the Node-B side. The UE can also be configured to use open loop or closed loop SATD mode if there is a full-power power amplifier in the UE or if the UE is not power limited even though it uses a half-power power amplifier. The active cells, i.e. the cells to which a UE is connected, belonging to the legacy Node Bs can correctly receive the data from the UEs in open loop or closed loop SATD mode.
UL MIMO in HSPA
For LTE, UL MIMO comprising of up to 4 transmit antennas in the UE is being specified, see RP-091430, UL multiple antenna transmission for LTE, work item, RAN#46. For WCDMA-HSPA, UL MIMO is a natural extension to UL beam-forming since UE that support BFTD will be equipped with transmit chains and there thus is not any hardware update for ULBF UE to support UL MIMO. In current WCDMA-HSPA Node B, there are typically two receive antennas. In future deployments, there can be additional receive antennas and more advanced receiver structures, This means that there can be even larger gain from UL MIMO in future deployments. For instance, advanced receivers such as enhanced receiver Type 3i and interference cancellation technology can be used in the Node B side when the related cost is decreased to an acceptable level.
A UL MIMO capable UE can be configured in default single TX antenna mode, open loop BFTD Mode, closed loop BFTD mode and UL MIMO mode. Among these modes, the first two TX antenna modes (single Tx and open loop TD) can be supported by all Node Bs. But the last two TX antenna modes cannot be supported by a cell associated with a legacy Node B. Just as a CLTD capable UE, the UL MIMO capable UE is likely to be configurable to operate in either open loop or closed loop SATD mode partly depending upon the UE implementation. For instance the UL MIMO capable UE may operate in SATD modes if it has a full-power power amplifier or if its power is not limited even when using a half-power power amplifier.
Deployment of UL MIMO and/or CLTD Capable Node B
A “universal” deployment of UL MIMO and/or CLTD capable Node Bs (also referred as new Node Bs hereinafter) can take a long time. For instance, new Node Bs may initially only be deployed in some hotspot or coverage limited areas where MIMO or BFTD technology is capable of providing significant capacity and/or coverage improvements, respectively. In such partial deployment scenarios the new Node Bs will be surrounded by legacy Node Bs, which cannot support UL MIMO or some CLTD transmissions. Furthermore, new Node Bs are likely to be deployed in areas where there is a need for a substantial performance improvement and/or need for higher capacity. Hence, partial deployment scenarios comprising of the mixture of the legacy Node B and new Node B (e.g. UL MIMO capable Node B:s) are also likely to exist at later stages.
A UE located in an area with mixed Node Bs (legacy Node Bs and New Node Bs) may be in soft handover with new and legacy Node B:s. This type of mixed deployment scenario with a UE in soft handover with new and legacy Node Bs is illustrated in FIG. 1. In summary, there can be mixture of new Node Bs, i.e. Node Bs capable of decoding uplink multi-antenna transmission with a new control channel format such as UL MIMO or some UL CLTD and legacy Node Bs, i.e. Node Bs not capable of such decoding in some border areas.
In such scenarios several problems can occur. For instance,                A legacy Node B may not be capable of receiving data streams from a UE configured in UL MIMO or some CLTD mode.        There is no soft handover gain from the cell in the active set that belongs to the legacy Node Bs for a UE configured in UL MIMO or some CLTD mode.        The handover from a cell belonging to a new Node B to a cell belonging to a legacy Node B of a UE in UL MIMO or some CLTD mode should be smooth and seamless        
A legacy Node B may not be able to control the transmit it power of the UE (if it cannot be part of the active set for a UE configured in SHO) by means of the overload indicator (transmitted over the relative grant channel). This may result in the undesirable event that an UL MIMO UE causes a severe interference level at the legacy Node-B.
Hence, there exist a need for new methods and devices providing improved performance in cellular radio systems with MIMO in the uplink.